It was established that the engine flameouts were caused by ice in the engine air inlet ducts lifting up as a solid sheet interrupting the airflow to the engines and causing them to flame-out. Two possible scenarios were established for the ice built up in the engine air inlet ducts. One scenario is that there was water in the engine air inlet ducts when the aircraft was removed from the hangar. This water went undetected during the inlet inspections and then froze into a solid sheet. The other scenario is that the engine air inlet ducts were clean when the engine inlet plugs were removed for engine start and that a sheet of ice formed in each engine inlet duct after engine start. The first scenario postulates that because the aircraft had arrived in Sydney in blowing snow conditions, snow/ice accumulated in the inlet duct before the aircraft was placed in the unheated hanger. Although this is likely, (either from in-flight accumulation or from after landing accumulation, or a combination of both) it was not verified that snow/ice was present. If snow/ice had accumulated, it would have melted because the temperature inside the hangar was slightly above freezing and the residual engine heat in the inlets would have raised the inlet temperature after the aircraft engines had been shut down. Water from the melted ice that would have normally drained out the inlet duct drain holes would have remained in the inlet because of the drain hole blockages. When the aircraft was removed from the hangar the next day, the water in the bottom of the inlet ducts froze. This water and/or ice went undetected by the flight crew or the ground handler when he installed and removed the inlet plugs. Thirty minutes after the aircraft was removed from the hangar the right engine was started and run for five minutes to heat the aircraft. Both engines were started 46minutes after the aircraft was out of the hangar. Of the total period outside the hangar before both engines were started, the plugs were estimated to have been in place for 20minutes, leaving the inlets exposed to the wind and blowing snow for 26minutes, with the right engine running for 5minutes of that. The heat transfer analysis concluded that, in conditions similar to those on the ramp on the day of the occurrence (temperature -1C, wind 10knots), inch of water could freeze in 30minutes. Given that any water in the inlet ducts would have been exposed (plugs removed) to the wind prior to engine start for approximately 10minutes with respect to the right engine, and slightly less than 30minutes for the left engine, the heat transfer analysis conclusion may not be completely appropriate for both engines. The left engine, however, would have been exposed to the -1C temperature for about 46minutes. Heat transfer analysis also concluded that the conditions on 03April2001, after engine start, were not conducive to inlet duct icing and consequently, the inlet icing could not have occurred as described by the crew unless there was a pre-existing, ground-accumulated ice sheet. Previous occurrences where it has been concluded that inappropriate ground handling procedures resulted in an ice/snow build-up in the engine inlets were also used to support the first scenario. The second scenario postulates that the inlet ducts were clear of water and ice prior to engine start and that ice developed in the inlet ducts after the engines were running. In this scenario, there is agreement that any ice present on arrival the night before would have likely melted after the aircraft was put in the hangar. There is disagreement, however, regarding the presence of ice or water in the inlet ducts prior to engine start, because the ground handler's direct visual and tactile inspection of the inlets showed them to be clear. Further, ice formation in flight is supported by the pilots' observations from the cockpit of ice forming in the inlet ducts after take-off, and the fact that there was no ice visible in the inlet ducts after the flame-outs. The incident of 05December2001 and other ACR in-flight-icing reports confirm that inlet duct icing can occur. Theoretical modeling was used to develop the conclusions in the first scenario. The second scenario relies on ACR personnel statements that no ice was present before engine start and that the proper ground and flight procedures were followed. While it is not possible to determine conclusively which scenario is accurate, the implications of either possibility are serious. To deal with the first scenario and ensure that inlet ice contamination on the ground will not result in an engine flame-out, the manufacturer introduced additional ground handling safety defenses. These procedures have been implemented by ACR. The second scenario, that the multiple in-flight engine flame-outs may have been caused by ice accumulation after the engines were started, cannot be discounted as a possibility. Therefore, even though engine ignition successfully re-started the engines on this occasion, appropriate follow-up action is required to ensure that the risk of significant in-flight ice accumulation causing flame-outs is adequately assessed.Analysis It was established that the engine flameouts were caused by ice in the engine air inlet ducts lifting up as a solid sheet interrupting the airflow to the engines and causing them to flame-out. Two possible scenarios were established for the ice built up in the engine air inlet ducts. One scenario is that there was water in the engine air inlet ducts when the aircraft was removed from the hangar. This water went undetected during the inlet inspections and then froze into a solid sheet. The other scenario is that the engine air inlet ducts were clean when the engine inlet plugs were removed for engine start and that a sheet of ice formed in each engine inlet duct after engine start. The first scenario postulates that because the aircraft had arrived in Sydney in blowing snow conditions, snow/ice accumulated in the inlet duct before the aircraft was placed in the unheated hanger. Although this is likely, (either from in-flight accumulation or from after landing accumulation, or a combination of both) it was not verified that snow/ice was present. If snow/ice had accumulated, it would have melted because the temperature inside the hangar was slightly above freezing and the residual engine heat in the inlets would have raised the inlet temperature after the aircraft engines had been shut down. Water from the melted ice that would have normally drained out the inlet duct drain holes would have remained in the inlet because of the drain hole blockages. When the aircraft was removed from the hangar the next day, the water in the bottom of the inlet ducts froze. This water and/or ice went undetected by the flight crew or the ground handler when he installed and removed the inlet plugs. Thirty minutes after the aircraft was removed from the hangar the right engine was started and run for five minutes to heat the aircraft. Both engines were started 46minutes after the aircraft was out of the hangar. Of the total period outside the hangar before both engines were started, the plugs were estimated to have been in place for 20minutes, leaving the inlets exposed to the wind and blowing snow for 26minutes, with the right engine running for 5minutes of that. The heat transfer analysis concluded that, in conditions similar to those on the ramp on the day of the occurrence (temperature -1C, wind 10knots), inch of water could freeze in 30minutes. Given that any water in the inlet ducts would have been exposed (plugs removed) to the wind prior to engine start for approximately 10minutes with respect to the right engine, and slightly less than 30minutes for the left engine, the heat transfer analysis conclusion may not be completely appropriate for both engines. The left engine, however, would have been exposed to the -1C temperature for about 46minutes. Heat transfer analysis also concluded that the conditions on 03April2001, after engine start, were not conducive to inlet duct icing and consequently, the inlet icing could not have occurred as described by the crew unless there was a pre-existing, ground-accumulated ice sheet. Previous occurrences where it has been concluded that inappropriate ground handling procedures resulted in an ice/snow build-up in the engine inlets were also used to support the first scenario. The second scenario postulates that the inlet ducts were clear of water and ice prior to engine start and that ice developed in the inlet ducts after the engines were running. In this scenario, there is agreement that any ice present on arrival the night before would have likely melted after the aircraft was put in the hangar. There is disagreement, however, regarding the presence of ice or water in the inlet ducts prior to engine start, because the ground handler's direct visual and tactile inspection of the inlets showed them to be clear. Further, ice formation in flight is supported by the pilots' observations from the cockpit of ice forming in the inlet ducts after take-off, and the fact that there was no ice visible in the inlet ducts after the flame-outs. The incident of 05December2001 and other ACR in-flight-icing reports confirm that inlet duct icing can occur. Theoretical modeling was used to develop the conclusions in the first scenario. The second scenario relies on ACR personnel statements that no ice was present before engine start and that the proper ground and flight procedures were followed. While it is not possible to determine conclusively which scenario is accurate, the implications of either possibility are serious. To deal with the first scenario and ensure that inlet ice contamination on the ground will not result in an engine flame-out, the manufacturer introduced additional ground handling safety defenses. These procedures have been implemented by ACR. The second scenario, that the multiple in-flight engine flame-outs may have been caused by ice accumulation after the engines were started, cannot be discounted as a possibility. Therefore, even though engine ignition successfully re-started the engines on this occasion, appropriate follow-up action is required to ensure that the risk of significant in-flight ice accumulation causing flame-outs is adequately assessed. It was determined that the engine flame-outs were caused by ice in the engine air inlet ducts lifting up as a solid sheet interrupting the airflow to the engines and causing them to flame-out. It could not be determined conclusively how the ice formed in the inlet ducts.Findings as to Causes and Contributing Factors It was determined that the engine flame-outs were caused by ice in the engine air inlet ducts lifting up as a solid sheet interrupting the airflow to the engines and causing them to flame-out. It could not be determined conclusively how the ice formed in the inlet ducts. Three of the four drain holes in the right engine inlet duct were completely blocked and the fourth was partially blocked, which increased the risk that water could pool and freeze in the duct.Findings as to Risk Three of the four drain holes in the right engine inlet duct were completely blocked and the fourth was partially blocked, which increased the risk that water could pool and freeze in the duct. Bombardier Aerospace published a revised ground procedure training guide in September2001. The main difference in this guide compared to the previous version (September2000) is that it contains a more detailed description of the areas to be inspected (tactile inspection of inlet duct area is added) and cleaned, and it suggests tools and methods for carrying out the inspections and cleaning procedures. Bombardier Aerospace has also provided instructions, Customer Special Installation (CSI) 826930, on enlarging the drain holes in the engine air inlet ducts. ACR has incorporated the revised procedures into their training program and SOPs. In addition, ACR has developed, for data collection purposes, an Engine Intake Ice Survey form. Flight crew complete this form anytime ice is detected in the engine air inlets. In conjunction with this program, ACR and Environment Canada have entered into a program which provides real-time monitoring of in-flight atmospheric conditions. Data from this program will be correlated with data from the ice surveys in an attempt to understand the conditions which lead to engine air inlet duct ice formation in order to develop appropriate icing avoidance procedures. As of 31 December 2001, the operator has received several completed Ice Survey forms and it was this survey that resulted in detection of the icing incident on the flight of 05December2001. ACR has commenced the installation of splitter angles, (designed by Bombardier Aerospace at the request of ACR [to install some kind of device in the inlet duct so that any ice sheet, regardless of how it got there, would not lift as a solid sheet] and provided to ACR as CSI 44022) in the engine nacelle lower cowl. ACR has installed this device in all of their DHC-8 aircraft. The purpose of the splitter angles is to prevent a single, solid sheet of ice from forming in the engine lower cowl. The company has also completed a program to enlarge the drain holes in the engine inlet ducts in accordance with CSI 826930. A TSB Aviation Safety Advisory was sent to Transport Canada on 17August2001 suggesting that this and previous occurrences involving DHC-8-100 engine flame-outs be reviewed to validate that the aircraft and engines (Pratt Whitney 120A) were performing acceptably under conditions for which they are certified. On 23 October 2001 Transport Canada responded to the safety advisory. Stated in the response were the following: Bombardier has developed extensive ground procedures for the upcoming winter2001/2002 and they will assist Air Canada Regional (ACR) to implement them [This activity was completed]. Bombardier will station a Field Service Representative in the Atlantic region this winter to ensure that the procedures are understood and to collect data in the ACR operating environment. TC Civil Aviation staff members are satisfied that ACR, Bombardier, and Pratt and Whitney Canada are working collaboratively to ensure that there is not a recurrence of the event experienced by ACR on 03April2001. TC is satisfied with the progress to date and will continue to monitor and support these efforts until the issue is resolved. This report concludes the TSB's investigation into this occurrence. Consequently, the Board authorized the release of this report on 03June2003. 1.All times are Atlantic standard time (Coordinated Universal Time minus three hours) unless otherwise noted.Safety Action Taken Bombardier Aerospace published a revised ground procedure training guide in September2001. The main difference in this guide compared to the previous version (September2000) is that it contains a more detailed description of the areas to be inspected (tactile inspection of inlet duct area is added) and cleaned, and it suggests tools and methods for carrying out the inspections and cleaning procedures. Bombardier Aerospace has also provided instructions, Customer Special Installation (CSI) 826930, on enlarging the drain holes in the engine air inlet ducts. ACR has incorporated the revised procedures into their training program and SOPs. In addition, ACR has developed, for data collection purposes, an Engine Intake Ice Survey form. Flight crew complete this form anytime ice is detected in the engine air inlets. In conjunction with this program, ACR and Environment Canada have entered into a program which provides real-time monitoring of in-flight atmospheric conditions. Data from this program will be correlated with data from the ice surveys in an attempt to understand the conditions which lead to engine air inlet duct ice formation in order to develop appropriate icing avoidance procedures. As of 31 December 2001, the operator has received several completed Ice Survey forms and it was this survey that resulted in detection of the icing incident on the flight of 05December2001. ACR has commenced the installation of splitter angles, (designed by Bombardier Aerospace at the request of ACR [to install some kind of device in the inlet duct so that any ice sheet, regardless of how it got there, would not lift as a solid sheet] and provided to ACR as CSI 44022) in the engine nacelle lower cowl. ACR has installed this device in all of their DHC-8 aircraft. The purpose of the splitter angles is to prevent a single, solid sheet of ice from forming in the engine lower cowl. The company has also completed a program to enlarge the drain holes in the engine inlet ducts in accordance with CSI 826930. A TSB Aviation Safety Advisory was sent to Transport Canada on 17August2001 suggesting that this and previous occurrences involving DHC-8-100 engine flame-outs be reviewed to validate that the aircraft and engines (Pratt Whitney 120A) were performing acceptably under conditions for which they are certified. On 23 October 2001 Transport Canada responded to the safety advisory. Stated in the response were the following: Bombardier has developed extensive ground procedures for the upcoming winter2001/2002 and they will assist Air Canada Regional (ACR) to implement them [This activity was completed]. Bombardier will station a Field Service Representative in the Atlantic region this winter to ensure that the procedures are understood and to collect data in the ACR operating environment. TC Civil Aviation staff members are satisfied that ACR, Bombardier, and Pratt and Whitney Canada are working collaboratively to ensure that there is not a recurrence of the event experienced by ACR on 03April2001. TC is satisfied with the progress to date and will continue to monitor and support these efforts until the issue is resolved. This report concludes the TSB's investigation into this occurrence. Consequently, the Board authorized the release of this report on 03June2003. 1.All times are Atlantic standard time (Coordinated Universal Time minus three hours) unless otherwise noted.